1. Field of the Invention
The invention relates to a thermoplastic crystallizable polyester or a composition comprising such a polyester. More specifically, the invention relates to a crystailizable polyester or crystallizable polyester composition which provides a gas barrier characterized by an oxygen permeability of from about 0.2 to about 4.9 cc-mil/100 .sup.2 -24 hr-atm at 23.degree. C. and relative humidity of 60% outside/100% inside, has a linear dimensional shrinkage of less than about 6% between 23.degree. C. and about 200.degree. C., and has an enthalpy of recrystallization greater than about -2.1 cal/g between about 100.degree. C. and about 170.degree. C. as determined by heating differential scanning calorimetry (DSC). The invention also relates to a method of making a polyester with the above-described characteristics, as well as to an article of manufacture comprising the present polyester and to a method of manufacturing such an article. Due to its high dimensional stability, the product of the invention is useful as a container for food products, particularly those that are cooked either by microwave or conventional ovens. Additionally, due to its low gas permeability, the product of the invention is useful for increasing the shelf life of food and beverage products at temperatures from about -60.degree. C. to about 215.degree. C.
2. Technology Review
Polyesters have heretofore been widely used in the food package industry, including blister packs for meats, containers for frozen foods, ovenable and microwavable ("dual ovenable") trays and carbonated beverage bottles. A major effort in such packaging applications has been directed toward reducing the gas permeability of the package, since a decrease in such permeability will lead to a longer shelf life of the food product, be it at frozen, refrigerated or ambient temperature storage. Another focus of great effort in the food packaging industry is the dimensional stability of the package over long periods of time at ambient temperature or upon heating of the contents in either microwave or conventional ovens. Of particular concern in this regard are the breakage of seals and warping due to excessive shrinkage of the container during retort or any of the thermal sterilization processes.
A number of prior patents have addressed the abovementioned concerns, for the most part individually. As a result, for example, polyester compositions that are particularly suitable as carbonated beverage containers due to there low gas permeability, are generally unsuitable for applications and processes requiring elevated temperatures, as these materials generally exhibit excessive shrinkage and warping at these higher temperatures. Examples of such prior activity include:
U.S. Pat. No. 4,560,741 discloses a polyester resin derived from a C.sub.2-8 diol, oxydiacetic acid and naphthalene dicarboxylic acid having improved resistance to gas permeability as compared to polyethylene terephthalate homopolymers.
U.S. Pat. No. 3,960,807 discloses a heat-set article comprising a polymeric crack-stopping agent and a nucleant as having a good dimensional stability and impact resistance.
U.S. Pat. No. 4,463,121 discloses thermoformed articles of partially crystallized polyethylene terephthalate and a polyolefin, as having improved impact resistance and high temperature dimensional stability.
U.S. Pat. No. 4,572,852 discloses crystalline polyethylene terephthalate/polyolefin article as having high dimensional stability.
U.S. Pat. No. 4,618,515 discloses a polyethylene terephthalate wide mouth bottle wherein the neck portion has higher thermal and strain crystallinity than the rest of the bottle, such that the neck is more resistant to shrinkage during a hot-fill process.
U.S. Pat. No. 4,535,025 discloses a biaxially-oriented, heat-set polyethylene terephthalate material with a density of over 1.4050 g/cc at 25.degree. C. as having improved gas barrier properties.
U.S. Pat. No. 4,282,277 discloses a biaxially-oriented thermoset polymer seamless conduit as having good low temperature tensile impact strength.
U.S. Pat. No. 5,003,041 discloses blends of polyethylene terephthalate and trans-4,4'-stilbenedicarboxylic acid as having improved gas barrier properties as compared to polyethylene terephthalate homopolymer.
U.S. Pat. No. 4,764,403 discloses a biaxially-oriented, heat-set, multi-layer article with an inner layer of polyethylene terephthalate, as having high barrier properties and improved thermal stability.
U.S. Pat. No. 4,874,647 discloses a composition of polyethylene terephthalate and bisphenol-A polycarbonate for use in a polyester laminate. The composition is disclosed as providing for improved mechanical strength for a heat resistant polyester.
U.S. Pat. No. 4,061,706 discloses a continuous melt process for thermoforming thermoplastic polymers, preferably polyamides.
U.S. Pat. No. 4,261,473 discloses a container made of thermoplastic resin, oriented in at least one direction, as having an oxygen permeability of lower than 5.times.10.sup.-11 cc-cm/cm.sup.2 -sec-cm Hg.
U.S. Pat. No. 4,469,270 discloses a shaped container of polyalkylene terephthalate with a crystallinity of at least 20 percent as determined by density measurement.
U.S. Pat. No. 4,996,269 discloses a thermoplastic resin of polyethylene naphthalate and a polyester elastomer and having a crystallinity from 10 to 40 percent as measured by density, as having high dimensional stability.
The conventional process for manufacturing polyester containers, herein referred to as a "glass-to-mold" process, has at least two heating steps; the first during production of the polyester source material by the supplier, and the second during shaping of the polyester into a container by the manufacturer. In the first step of the conventional process, the source polyester material is cooled as it is formed into pellets, rolls, sheets or other shapes suitable for shipping, storage and subsequent processing into articles of manufacture. In many processes, such as that for producing amorphous polyethylene terephthalate (A-PET), the cooling of the material from the molten state is at a sufficiently rapid rate so as to thermally quench most of the dynamic crystallization of the polymer and thus produce an undercrystallized material. In addition, thermal gradients may arise in the polyester during heating and cooling. The material stresses due to these thermal gradients then become frozen in the ambient temperature material. Such stresses due to thermal gradients are referred to herein as thermally induced stresses.
In the second step of conventionally producing articles made of polyesters, the pellets, sheets, etc. of polyester made in the first step are reheated until the material reaches a recrystallization onset temperature. At this point recrystallization of the material begins. Increased crystallinity is desirable in a product as it increases the melting temperature of the polyester so as to allow it to be used in a conventional oven for reconstitution.
Recrystallization upon reheating of a crystallizable polyester may be due to the further growth of existing crystals in the material or to the formation of new crystals, or both. The recrystallization onset temperature of a polyester may be easily detected by heating differential scanning calorimetry as that temperature at which the exothermic recrystallization reaction begins. The recrystallization onset temperature of a polyester material as determined in this way is localized to a temperature between the glass transition temperature and the melting temperature of the material and is dependent upon polymer chain length and composition, and the heating rate.
In the conventional thermoforming process, the heated polyester is then maintained at or near the recrystailization onset temperature until the desired degree of crystallinity is achieved, after which the material is molded and rapidly cooled. During glass-to-mold thermoshaping, additional undesirable thermal stresses may also be introduced into the polyester article at this cooling step.
As a result of the glass-to-mold manufacturing process, upon reheating of a conventional polyester article to the recrystallization onset temperature of the material, the undercrystallized material then again begins recrystallization. Such a situation presents a serious drawback to articles made by conventional processes, particularly those used as containers for the storage of foodstuffs and products that are meant to remain in sterilized state or are meant to be reheated to at or above the recrystallization onset temperature. Because of the recrystallization of the container on subsequent reheat, the additional shrinkage may result in not only product deformation, by also seal breakage or complete product failure as a container or barrier. Thus, a substantial number of previously sterilized items within a polyester container made by conventional processes must be discarded, resulting in a substantial amount of waste.
It is also known that amorphous phase polymer chains may be axially or bi-axially oriented by applying force, in either one or two directions, respectively, to the polymeric material while it is in a semi-fluid state, usually above the glass transition temperature (Tg) of the polymer. To achieve such a polymer chain orientation, a mechanical force is continuously applied as the polymeric material is cooled to below its glass transition temperature (Tg) It is known that such orientation of the polymers in a material decreases the gas permeability of the material as compared to a non-oriented polyester. For polyethylene terephthalate, such orientation reduces the oxygen permeability at 0% humidity from about 10 cc-mil/100 in.sup.2 -day-atm to 5 cc-mil/100 in.sup.2 -day-atm. However, chain orientation by this method results in mechanical stresses becoming frozen in the ambient temperature material. Heating of the oriented material to near or above the Tg results in the release of the physically induced stress contained therein, with the result that the material undergoes substantial dimensional distortion. Such distortion may also occur over long periods of time at ambient temperature. Thus, although the gas barrier properties of oriented materials make them particularly suitable for uses such as carbonated beverage containers, these same materials are wholly unsuitable for use where temperatures near or above the Tg of the material will be encountered. In particular, such oriented materials are unsuitable for use in retort, hot-fill and high temperature sterilization processes.
The thermally and physically induced stresses in a polyester result in a molecular imprinting of the past thermal history of the material, generally referred to as "memory". Such memory can only be completely removed from the material by reheating the polyester to above the melting temperature for a sufficient time to bring the material to a completely amorphous, molten state. Such high temperatures, however, are not used in the glass-to-mold thermoforming process. As a result, articles made in this way retain much of the stress introduced in their process of manufacture.
Thus, another drawback to articles made by the glass-to-mold process is that upon reheat some of the thermally and mechanically induced stresses frozen in the polyester are relieved, which may result in severe dimensional changes in the molded material. The greater degree of thermally and mechanically induced stresses imprinted in a article, the greater the potential for deformation therein upon reheating. As with the dimensional changes associated with recrystallization, distortions due to stress release can cause container seal failure and seam failure, with the resultant loss of sterility of the foodstuffs contained therein. Such stress release may additionally cause undesired warping during retort, hot-fill and high temperature sterilization processes. As meant within this disclosure, the sum of the dimensional changes upon heating a polyester due to recrystallization and stress releases are referred to as the thermal dimensional stability of the polyester. Such a characterization of the thermal dimensional stability of a polyester may be conveniently expressed as the percent change in either linear, planar or volume dimensions, as appropriate, for a polyester article, that results from elevating the temperature of the article from about -60.degree. C. to a temperature just below the onset of melting of the article. For example, for polyethylene terephthalate, this temperature range is from -60.degree. C. to about 200.degree. C.
An additional drawback to article of glass-to-mold manufacture is that since these polyesters retain a memory and may be undercrystallized, the use of scrap material derived in the manufacture of these articles is made less than highly desirable for inclusion in heat-based reclamation processes, as the stress memory and sub-optimal crystallinity will become incorporated in the recycled material, thereby conferring an undesirable thermal instability on the recycled article.
A further undesirable consequence of the glass-to-mold process of manufacturing a polyester article is the additional energy that must be expended to reheat the polymer prior to the shaping process. Such additional heating is both time consuming and energy-inefficient.
In light of the above considerations, there clearly exists a need for a polyester that is dimensionally stable at both low and high temperature, while at the same time provides an improved gas barrier. Preferably, such a product would be sterilizable by temperature, pressure, chemical and/or radiation methods. Still more preferably, such a polyester would be compatible with use for containing foodstuffs, beverages, and the like, and would be both microwavable and ovenable at high temperatures. It would additionally be advantageous for such a product to be recyclable, so as to both decrease the cost of manufacture and provide a product that is environmentally sound.